Transmission line directional coupler



June 4, 1957 l Rfw. PETER 2,794,958

TRANSMISSION LINE DIRECTIONAL coUPLER Filed oct. s1, '1951 '2sheets-sheet 1 finger/0N T 1 L IND/Mraz f IIIIII' ATTORNEY June 4, 1957R. w. PETER 2,794,958

` TRANsmssIoN LINE DIRECTIONAL coUPLER Filed Oct. 31, 1951 2Sheets-Sheet 2 V'Il/IA ilrlllz infill/lll; IIIIIIIIA rlllli/ FEO/VSOI/ECE INVENTOR Rolf Wren-r BY im ATTORNEY United States Patent Mice2,794,958 TRANSMISSION LINE DIRECTIGNAL COUPLER Rolf Walter Peter,Princeton, N. J., assgnor to Radio Corporation of America, a corporationof Delaware Application October 31, 1951, Serial No. 254,048

17 Claims. (ci. ssa- 10) The present invention relates to signal wavetransmission systems and particularly to directional couplerarrangements especially well suited for coaxial transmission lines andfor measuring signal reections on the line from a load.

A compact directional coupler arrangement is frequently desirable inwhich the proportion of power transfer is large compared to the space orto the length of line utilized for coupling. Other desirable qualitiesof directional couplers are that the coupling be matched, in which eventreflections of incident power from the coupling structure arelsubstantially absent; and broad banding, that is, the couplingcharacteristics including power transfer, do not vary over a broad bandof frequencies about an operating frequency. Previous coaxialtransmission line directional coupler networks have involved ring or ratrace arrangements, and the like.

Generally a directional coupler arrangement includes a linear passivenetwork coupling between four transmission line arms. Energy incident onthe network from one arm is.divided between vtwo of the other arms butis notcoupled directly. to the fourth. Similarly, energy incident fromthe fourth arm is divided between the second and third arms, and is notcoupled to the rst arm. v Ideally there are no reections and the armslook into matched impedances. Consider the first and second arms .aspart of one transmission line, and the third and fourth as part ofanother. first arm in a direction toward the network (thus travelling inone direction in one transmission line) is partially coupled to thethird arm, and travels in this third arm in a direction away from thenetwork. Similarly, energy vcoupled into the fourth .arm incident fromthe second a reverse direction in the one trans- (thus travelling inmission line) travels away from the network in the fourth arm.Therefore, the direction of energy flow in the ysecon-d transmissionline is determined selectively by the'direction of energy flow in theone transmission line. The converse is also true.

It is an object of the invention to provide a novel directional couplerarrangement for coaxial transmission lines.

It is another object of the invention to provide a coaxial transmissionline directional coupler arrangement that is compact, easy to construct,and broadbanded.

It i-s another object of the invention to provide a novel compact powermonitoring device for coaxial transmission lines.

In accordance with a preferred embodiment of the invention, a pair ofcoaxial transmission line sections are coupled in a directional couplernetwork over a lengthA in which an intermediate conductor serves both asthe outer conductor of the inner line section and as the inner conductorof the other line outer section. The coupling is provided by a helicalslot in the said intermediate conductor through which theelectromagnetic fields of one line .are coupled to those of the other. Acoaxial line arm is connected at one end to supply energy. At the Thenenergy incidentin the i Patented June 4, 1 957 teristic impedance of theinner line section and' its ex-v tensions may be maintained constantthroughout to aid in securing broad-banded matching.

In other embodiments of the invention, preferred for power monitoring,the output or energy withdrawal end of one line is terminated with anabsorbent termination and the other end has a detector crystaltermination forl monitoring power output or for monitoring for theamount of reflected power, to test for matched conditions of a load.v

The foregoing objects and other objects, advantages, and novel featuresof the invention will be more apparent from the following description,in which like reference numerals refer to like parts, and in which:

Fig. 1 is .a longitudinal cross-sectional view of one embodimentaccording to the invention which is broadbanded in operation because ofconnections to the outer of two coaxial transmission lines coaxial witheach other;

Fig. 2 is a longitudinal cross-sectional fragmentary view of anotherembodiment of the invention in which the characteristic impedance ofboth the inner and outer of two coaxial transmission line sectionscoaxial with each other is maintained the same over the coupling region;

Figs. 3 and 4 are longitudinal cross-sectional views of power monitoringarrangements according to the inventionl mission lines couples coaxialtransmission line arms 10,y

12, 14, and 16. The network 8 includes an inner continuous conductor 18,an intermediate conductor 20, and an outer conductor 22. Theintermediate conductor serves as the inner line outer conductor of thesection 8 and also as the outer line inner conductor of the section 8.The inner conductor 18 of the inner line is extended at each enddirectly into the inner conductors respectively of coaxial line arms 14and 16. The section intermediate conductor 20 is extended at each of itsends respectively ldirectly into the outer conductors of arms 14 and 16.The arms 10, 12 are coupled to the section V8 respectively by broad bandstub supports 24 and 26. Thev stub support 24 is connected betweenintermediate conductor 20 near one end of section 8 and the innerconductor of arm 10. The stub support 26 is connected betweenintermediate conductor 20 near the other end of section. 8 and the innerconductor of arm 12. At the end-s of section 8, a quarter wavelengthrespectively from the points of connection of stub supports 24 and 26,the section outer conductor 22'is short circuited to the sectionintermediate conductor 20. Each of the stub supports 24 and 26 is of thetype illustrated on page 58 (Fig. III-42) and described in theassociated text of Microwave Transmission Design Data, Publication No.23-80 of Sperry Gyroscope Co., Inc. This type of 3. right angle lstubsupport is hereinafter termed a lright angle broadband stub support.`

A helical slot 28 is cut in the section intermediate conductor 20between the pointsof connection thereto of the stub supports 24 and 26.The-pitch, width, and number of turns of the slot 28 are selected toprovide theldesired division of power-or coupling 'from energy incident:at the operating frequency band in one of the Apairof arms 10,12.through the section to one of the arms '16, 14 respectively, theremaining energy being transferred to the other arm 10,12 of the pair;

In the preferred arrangement, tthepitch of the slot is preferablyinfinite at each end and gradually mergesinto a helical slot of constantpitch, as illustrated. This preferred arrangement provides a broaderband of operating frequencies over which `the desired division of poweris secured than if the longitudinally slotted infinite pitch portion ofthe slot were not employed.

In operation, let energy be supplied from a source, as indicated on thedrawing, to waveguide 10. This energy at the junction of lines andsection 8 sees an open circuit to the left beyond the end of section 8as viewed in Fig. 1, due to the quarter wavelength short-circuitedportion of conductors and 22. Therefore, the energy ows to the right.Some of the energy is supplied through the helical slot 28 which allowscommunication between the inner coaxial line section 18, 20 and theouter coaxial line section 20, 22.` The energy thus supplied to theinner coaxial line section 18, 20 ilows to the right to a load A(notshown). The remaining energy in the outer coaxial line section 20,22 `flows to the stub supported right-angle bend 26. t The energy herefaces an apparent open circuit to the right, due again to the shortcircuit of conductor 22 to 20 at this end. The only remaining pathtisout coaxial line 12 to a load B,(not'sliown). The actionqof the rightangle stub supports gives broad banding energy transfer as described inthe said volume `on Microwave Transmission Design Data.

The amount of energy coupled from the outer coaxial linesection 20, 22`to the inner coaxial line section 18,` 20 through the'helical slot 28depends on the axial length, pitch, and width of the slot 28. These maybe varied empirically to secure a desired proportion of energy transfer.

In Fig. 1 it may be assumed that it is desired to divide the energyapplied from the sourceequally between two loads, A and B. Then anyenergy detected at line 14 mustbe reflected energy, returned from theloads. Here it is shown how to couple the loads A and B to the source,preferably equally if the section `8 is arranged to couple 50% of theenergy from line 10 to line 12 and 50% to line l16. If `desired adetector crystal with ammeter 32 is used as a reflection indicator.`

It is apparent that the ammeter will indicate current only when at leastone of loads A andB is` not matched, and will vshow zero current onlywhen both loads A and B are matched to their respective lines 16 and 12.Notice that lines 10 and 14 are decoupled, and that `lines 12 and 16 aredecoupled. The stubs 24 `and 26 of diameter enlarged respectively overthat of the inner conductors of lines 10 and 12 give a broad bandingeffect for the stub supported right angle bend. This effect is conjoinedwith that due to the tapering of the` sections.

In order to enhance the matching of the transmission line arms 10, 12,14, and'16 to the section 8, section 8 may have a central portion `asshown in Fig. 2. The section intermediate conductor -20 is slottedwithahelical slot 28 as in Fig. l. However the section 8-outer condoctor-isreplaced by an outerconductor 34 which has a portion'34a of reducedinternal diameter about the slotted portion of the section intermediateconductor 20. Also, the section inner conductor 18 of Fig. 1 isreplacedfby a. section inner conductor `36 having aportion 36a ofenlarged diameter inside the slotted portion of the section intermediateconductor.

By proper selectiontof the diameters of the enlarged diameter sectioninner conductor portion 36a, the characteristic impedance for thesection inner line may be maintained constant throughout the length ofsection 8. By proper selection of the diameters of the reduced diametersection outer conductor portion34a, the characteristic impedance of thesection outer line `may be maintained constant throughout the length ofsection 8. It is desirable that the characteristic impedance Zo of theportion with slotted conductor remain equal to the characteristicimpedance Z1 of the unslotted conductor. Let 1r=S/L, where S is totallength of the slot, and L is the axial length of the slot. It can beshown that if the decrease in spacing between conductors is such thatthe capacitance C0 per unit of line length along the portion of linewhere one of the conductors is formed with a helical slot is increasedby a factor 17, then Za is made equal to Z1. The impedances Zo' and Z1of the inner line can be made. equal in a similar manner.

If this method of equalizing impedances is employed, in the sectionshown the device is thereby made even more broad banded as hereinbeforementioned.

Referring to Fig. 3, a section 8 is shown having inner and outer coaxialtransmission lines 20, 36 and 20, 34 respectively, coaxial with eachother and similar to that shown in Fig. 2. At one end, the section 8 hasthe section outer transmission line 20, 34, extended and joined to acoaxial transmission line 38 leading from a source (not shown). Att theother end, the section outer line 20, 34 i's joined to and terminated bya resistive or iabsorptive termination 40. At this other end, thesection 8 inner coaxial transmission line has an extension 42 leading toa load. At the one end, the section inner line 20, 36 is terminated. bya crystal 30 in a suitable holder (not shown) inserted between the innerand outer conductor. At the other end, a pair of D. C. leads 44 is takenfrom the conductor ofthe load transmissionline 36, 42. AniR. F. choke,R. F. C., `keeps A. C; `currents out of meter 32. The crystal by-passcapacity is not-shown in the various figures.

In operation, suppose the slot affords a'50% coupling between the innerand outer coaxial lines 20, 36 and 20, 34. Then half the energy from thesourceis coupled to the termination 40; half is coupled to the load byline 42.,

It" the load is matched, there is no reflected energy to I. be rectifiedby crystal 30 and no D. C. current ow in meter 32. If the load is notmatched, however, `one-half the returned radio frequency energy isreturned `to the source, and one-half is returned to crystal 30,` whichrecties the R. F. energy incident on it, `and the rectified D.C.`currents are read in the meter 32. Therefore, the

i axial with` each other.

meter 32 indicates whether or not the load is matched to the load line42.`

, Referring to Fig. 4,a section8 has inner `and outer coaxial linesections 18, 20 and 20, 22 respectively co- Transmission line 14 from asource acts as one arm of the directional coupler section and feeds thesection outer coaxial line 20, 22 end-to-end. Transmission line l16leads toa load and acts as another arm of the directional coupler and isconnected end-toend to the other end of section outer line 20, :22 `fromthe end connected to source line 14. The section inner line 13, 20 isterminated at one end in a matched crystal arrangement including crystal30 and at the other end in matched resistive load 46. The endterminations of the section inner line 18, 20 `act as the third andfourth arms. It will be apparent from whathas been heretofore said, thatcrystal v30 is coupled to reliections from the load and decoupled fromenergy from the source. The load 46 is coupled to energy from the sourceand not to energyfrom theload. Preferably in this arrangement,l thecoupling coefficient is small. Reflections from the load are detectedyand measured by current ow in the crystal 30 and meter 32 circuit.

aree-95e Referring to Fig. 5, the source line 14 is connected to thesection inner line 18, Z and the load line 16 is connected to theSection inner line 18, 20. The section outer line 20, 22 is terminatedat the end coupled tothe source' in a resistive matched termination 40.The end of section outer line 20, 22 coupled to the loadretlections isterrninated in a crystal arrangement.

In Fig. 5, the directional coupledrcoupling through theV slot 28 ispreferably small, say 2 or 3% as in Fig. 4. The crystal 30 and meter 32circuit is such'that the crystal leads to the meter do not interferewith energy passage from source to load, `asthey need .not cross thesection line through which energy passes from sourceto'load.

In Fig. 6, energy from source` line 14 passes to the section inner line18, 20. The 'number of turns `and length of helical slot 28 is such thatthe percentage of coupling is preferably substantially greater than 50%.Line 16 is coupled to receive energy from line 14 through the sec-` tionouter line 20, 22. The crystal termination arrangement is similar tothat in Fig. 5. It is clear that the crystal is de-coupled from energyincident from the source, and is responsive only to energy returned fromthe load.

It will be apparent that there is disclosed herein a particularlydesirable, compactarrangement for detecting 'and measuring reilectionsfrom a load, and a particularly useful directional coupler arrangement,especially for applications where large coupling is desired.

What is claimed is:

1. A directional coupler comprising a section of inner and outer coaxialtransmission lines coaxial with each other, said outersection having acentral portion and portions next adjacent tosaid central portion, theinner of said section lines having an inner conductor, the outer of saidsection lines having an outer conductor, and said lines having incommonan intermediate conductor serving as the inner line outerconductor and the' outer line inner conductor, said intermediateconductor having a helical slot therein throughout said section centralportion aording energy communication between said section inner andouter lines, the said outer line outer conductor having a diameterthroughout said central portion reduced under its said diameter in saidadjacent portions.

2. A directional coupler comprising a section'of inner and outer coaxialtransmission lines coaxial with each other, said inner section having acentral portion and portions next adjacent to said central portion, theinner of said section lines having an inner conductor, the outer of saidsection lines having an outer conductor, and said lines having in commonan intermediate conductor serving as the inner line outer conductor andthe outer line inner conductor, said intermediate conductor having ahelical slot therein throughout said section central portion affordingenergy communication between said section inner and outer lines, thesaid inner line inner conductor having a diameter throughout saidcentral portion enlarged over its said diameter in said adjacentportions.

3. A directional coupler arrangement comprising a section of inner andouter coaxial transmission lines coaxial with each other, said sectionhaving a central portion vand portions next adjacent to said centralportion, the inner of said section lines having an inner conductor, theouter of said section lines having an outer conductor, and said lineshaving in common an intermediate conductor serving as the inner lineouter conductor and the outer line' inner conductor, said intermediateconductor having a helical slot therein throughout said section centralportion aiording energy communication between said section inner andouter lines, the said outer line outer conductor having a diameterthroughout said central portion reduced under its said diameter in saidadjacent portions, and the said inner conductor having a diameterthroughout said central portion enlarged over its said diameter in saidadjacent portions.

4. A directional coupler arrangement comprising a section of inner andouter coaxial transmission lines coaxial with each other, the inner ofsaid section lines having ari inner conductor, the outer of said sectionlines having an outer conductor, Iand said section lines having incorn-l mon for a portion an intermediate conductor serving as the innerline outer conductor and the outer line inner conductor, saidintermediate, conductor having a helical slot therein throughout saidportion thereof to afford energy communication between said sectioninner and outer lines, one of said lines having `an extension beyond oneend of said portion, said `one line having a ratio of inner diameter ofouter conductor to outer diameter of inner conductor in said portionthat is less than the like ratio for said extension.

5. The coupler arrangement claimed in claim 4, the other of said lineshaving an extension beyond an end of said portion, said other linehaving-a ratio of inner diameter of outer conductor to outer diameter ofinner conductor in said portion that is less than the like ratio forsaid other line extension.

6. The coupler arrangement claimed in claim 4, said one line having'another extension beyond the other end of said portion, said ratio forsaid one line being also less than the like ratio for said otherextension.

7 A directional coupler arrangement comprising a sectionrof inner andouter coaxial transmission lines coaxial withl each other, the inner ofsaid section lines having an inner conductor, the outer of said sectionlines having an outer conductor, and said section lines having in commonfor a portion thereof an intermediate conductor serving as the innerline outer conductor and the outer line inner conductor, saidintermediate conductor having a helical slot therein throughout saidportion to atord energy communication between said section inner andouter lines, each of said lines having extensions beyond each end ofsaid portion, said outer line having a ratio of inner diameter of outerconductor to outer diameter of inner conductor in said portion that isless than the like ratio for each of said outer line extensions, andsaid inner line having a like ratio in said portion that is less thanthe like ratio for said inner line extensions.

8. A coaxial line including an outer conductor and an inner conductor,one of said conductors being formed as a helix along one portion of itsextent; and the spacing between said two conductors being reduced to asuicient extent along the length of said helix to improve the impedancematch between the portion of the coaxial line which includes the helixand the remainder of the line.

9. A coaxial line including an outer conductor and an inner conductor,one of said conductors being formed as a helix along at least oneportion of its extent; and the distributed capacitance between said twoconductors along the portion of the coaxial line which includes thehelix being increased an amount suflicient to render its characteristicimpedance substantially equal to that of the remainder of said line.

l0. A transmission line including a pair of substantially parallelconductors, one of said conductors being formed as a helix along oneportion of the extent of said one conductor, and the spacing betweensaid pair of conductors being reduced to a sufficient extent along theportion of the line in Which the helix is located to render thecharacteristic impedance of said line substantially uniform throughoutits entire length.

ll. A coaxial line including an outer conductor and an inner conductor,said outer conductor being formed with a helical slot along at least oneportion of its extent, land the inner conductor of said line beingincreased in diameter along the portion of the line in which the helicalslot is located an amount suflicient to make the characteristicimpedance of said portion of said line substantially equal to that ofthe remainder of said line.

l2. A coaxial line including an outer conductor, and an inner conductor,said inner conductor being formed as a helix along at least one portionof its extent, and the inner diameter of said outer conductor beingreduced` the `distributed capacitance per unit of length f the sec.

tion of coaxial line in which the helix is located being increased `by afactorn, where S "rr and S is the extended length of the helix,` and Lthe axial length of the helix.

14. In combination, two coaxial lines including a rst conductor servingas the inner conductor of one line, a hollow `third conductor coaxialwith `said rst conductor serving as the outer conductor of the otherline, and a hollow second conductor coaxial with said first conductorand intermediate the first and third conductors serving as the innerconductor of said other line and the outer conductor of said one line,said hollow second conductor being formed with a helical slot along oneportion of` its extent, and the diameter of the trst conductor being in.

creased along the portion thereof adjacent the helical slot au amountsuicient to make the characteristic impedance of said one linesubstantially uniform throughout its length, and the inner diameter ofthe outer conductor along the portion thereof adjacent said helical slotbeing reduced an amount suicient to make the characteristic impedance ofsaid other line substantially uniform throughout its length.

15. A coaxial line including an outer conductor and an inner conductor,one of said conductors being formed as a helix coextensive `with theremainder of said one conductor along at leastlone portion `of itsextent, the pitch of said helix being substantially greater. at Ian endthereof coextensive with the remainder of said one conductor than thepitch of the helix in the center portion thereof; and the impedance ofthe section of line in which the conductor is a helix beingsubstantially equal to that of the remainder. of the line.

16. A coaxial line including an outer` conductor and an inner conductor,at least a part of said inner conductor being formed as a helix; and theimpedance of the section of line in which said inner. conductor is ahelix being substantially equal to that of the remainder of the line.

17. A coaxial line including an outer conductor and an inner conductor,at least a part of said outer conductor being formed as a helix; and theimpedance of the section of line in which said outer conductor is ahelix being substantially equal to that of the remainder of the line.

References Cited inthe file of this patent UNITED STATES PATENTS2,524,857 Seeker Oct. 10, 1950 2,580,678 Hansen et al. Ian. l, 19522,588,832 Hansell Mar. 1l, 1952 2,705,305 Bailey Mar. 29, 1955 i FOREIGNPATENTS `621,167 Great Britain Apr. 5, 1949 OTHER REFERENCES Proceedingsof the I. R. E, vol. 34, No. 10, October 1946,. page 792. (Copy inDivision 51.)

Moreno: Abstract of S; N. 605,417.0. G. vol. 640, nage 343, publishedNov. 7, 1950.

